The separation of air by rectification is very well known indeed. Rectification is a method in which mass exchange is effected between a descending stream of liquid and an ascending stream of vapour such that the ascending stream of vapour is enriched in a more volatile component (nitrogen) of the mixture to be separated and the descending stream of liquid is enriched in a less volatile component (oxygen) of the mixture to be separated.
In particular, it is known to separate air which has been cooled in a main heat exchanger in an arrangement of rectification columns comprising a higher pressure column and a lower pressure column. An initial separation is performed in the higher pressure column and as a result an oxygen-enriched liquid fraction is formed at its bottom and a nitrogen vapour fraction at its top. The nitrogen vapour fraction is condensed. A part of the condensate provides reflux for the higher pressure column and another part of the condensate provides reflux for the lower pressure column. A stream of oxygen-enriched liquid is withdrawn from the higher pressure column and is passed through an expansion device, normally a valve, into the lower pressure column. Here it is separated into oxygen and nitrogen fractions which may be pure or impure. Nitrogen and oxygen products are typically withdrawn from the lower pressure column and are returned through the main heat exchanger in countercurrent heat exchange with the first stream of compressed air. It is conventional to sub-cool the oxygen-enriched liquid stream upstream of the expansion device by indirect heat exchange with a nitrogen gaseous product stream withdrawn from the lower pressure column. Such sub-cooling reduces the amount of flash gas that is formed on expansion of the oxygen-enriched liquid stream. As a result, higher reflux ratios can be obtained in those regions of the lower pressure column below that at which the oxygen-enriched liquid stream is introduced, thereby facilitating the efficient operation of the lower pressure column. In addition, the sub-cooling has the effect of raising the temperature of the nitrogen product stream passing through the sub-cooler. This tends to have the benefit of reducing temperature differences in the main heat exchanger between air streams being cooled and product streams being warmed, and thereby leads to more efficient heat exchange. Nonetheless, the addition of a sub-cooler does add to the complexity of the air separation plant.
EP-A-0 848 220 shows, for example, in FIG. 8 an air separation plant in which the oxygen-enriched liquid stream taken from the higher pressure column is sub-cooled in the main heat exchanger. U.S. Pat. No. 5,275,004 discloses employing the main heat exchanger to perform the function of the reboiler-condenser that normally places the top of the higher pressure column in heat exchange relationship with the bottom of the lower pressure column. It is further disclosed in U.S. Pat. No. 5,275,004 that where the process comprises sub-cooling a liquid process stream in a sub-cooler, the sub-cooler's heat exchange service can be performed in the main heat exchanger.
It is an aim of the present invention to provide a method that enables a simplification of an air separation plant to be made without necessitating an undue loss of operating efficiency.